Abrading machines

ABSTRACT

THE SPECIFICATION DISCLOSES AN ABRADING MACHINE IN WHICH AIR AND ABRASIVE ARE DIRECTED AT A WORKPIECE IN A HINGED, TWO-PART TUBULAR BLASTING CHAMBER, THE AIR AND ABRASIVE BEING DIRECTED AT THE WORKPIECE BY NOZZLE ASSEMBLIES WHICH ARE ARRANGED SO THAT THEIR OUTLETS INTO THE CHAMBER LIE ON TWO OR MORE BELICES HAVING CONCIDENT AXES PARALLEL TO THE LONGITUDINAL AXIS OF THE CHAMBER BY A DISTANCE WHICH IS SMALL COMPARED WITH THE TOTAL LENGTH OF THE CHAMBER. THE NOZZLE ASSEMBLIES MAY HAVE THEIR INNER ENDS IN A PROJECTION. FUNNEL-SHAPED COLLECTORS MAY BE PROVIDED AT THE ENDS OF CHAMBER TO LEAD WIRE OR THE LIKE THROUGH THE CHAMBER. CLOSURE FLAPS MAY BE PROVIDED AT THE CHAMBER ENDS TO MAINTAIN THE VACUUM AS THE WIRE ENTERS OR LEAVES THE CHAMBER.

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3,611,640 ABRADlNG MACHINES Stewart lves Ashworth, Crimond, England, assignor to Abrasive Developments Limited, Solihull, Warwickslrire, England Filed Aug. 19, 11969, Ser. No. 851,354! Claims priority, application Great Britain, Dec. 17, 1968, 59,973/68 lint. Cl. 3245c 3/04 U.S. Cl. 5ll8 Claims ABSTRACT OF THE DISCLOSURE The specification discloses an abrading machine in which air and abrasive are directed at a workpiece in a hinged, two-part tubular blasting chamber, the air and abrasive being directed at the workpiece by nozzle assemblies which are arranged so that their outlets into the chamber lie on two or more helices having coincident axes parallel to the longitudinal axis of the chamber by a distance which is small compared with the total length of the chamber. The nozzle assemblies may have their inner ends in a projection. Funnel-shaped collectors may be provided at the ends of chamber to lead wire or the like through the chamber. Closure flaps may be provided at the chamber ends to maintain the vacuum as the wire enters or leaves the chamber.

BACKGROUND OF THE INVENTION Field of the invention This invention relates to apparatus for abrading a surface on a workpiece with a mixture of abrasive and air.

Description of the prior art In US. Pat. No. 3,286,406 issued Nov. 22, 1966 there is described apparatus for abrading a surface on a workpiece comprising a blasting chamber, a primary air inlet conduit connected at one end to the blasting chamber, means for entraining abrasive in the primary air flowing through the conduit upstream of its entry into the blasting chamber, aperture means in the wall of the chamber to be closed by the workpiece, means for applying suction to said chamber to draw primary air and abrasive into the chamber and remove air and spent abrasive from the chamber when said aperture means is closed by the workpiece, means for directing abrasive entering the chamber against said surface and means for restricting the entry of secondary air into the chamber around the periphery of the aperture means when the latter is closed by the workpiece. Hereinafter such apparatus is referred to as being of the type specified.

SUMMARY OF THE INVENTION According to the invention in apparatus of the type specified the blasting chamber is tubular and formed in two generally channel-shaped parts hinged together, the aperture means are at opposite ends of the chamber and the means for directing abrasive entering the chamber against the workpiece comprises a plurality of nozzle assemblies which are arranged so that their outlets into the chamber lie on at least two helices having coincident axes parallel to the longitudinal axis of the chamber, the helices being spaced apart axially of the chamber by a distance which is small compared with the total length of the chamber and each nozzle being directed at the axis of the helix on which the nozzle outlet lies, each helix comprising at least one full turn.

Where the nozzle assemblies have been mounted so that their outlets are on a single helix, we have found that when treating wire with such apparatus it sometimes happens that part of the surface of the wire is missed. This is because there is a twist in the wire as it comes off the coil and with a coil having a diameter of, for example, three feet the wire twists through every nine feet so that with the nozzle assemblies arranged with their outlets on a single helix it is sometimes possible to miss part of the surface of the wire due to this twist. We have found that with the outlets of the nozzle assemblies arranged on two or more helices spaced axially of the chamber by a small distance the area of the blast pattern is increased and there is considerably less chance of a portion of the wire treated being missed. Preferably, the nozzle assembly outlets on one helix lie opposite to the spaces between adjacent similar outlets on the other helix or other helices.

A further problem which may arise in using apparatus of the type specified having a tubular chamber as described for treating wire is that if the wire breaks it may damage the outlet ends of the nozzle assemblies which project into the chamber. Moreover, if an attempt is made to feed the wire through the chamber without opening the same, as may be possible with heavier Wires, the wire may get entangled with the outlet ends of the nozzle assemblies.

Preferably, therefore, the nozzle assemblies are mounted in one or more inward helical projections on said parts so that the outlets of said nozzle assemblies do not extend from said projection or projections.

There will normally be a single helical projection on each of said parts, the nozzle assemblies being mounted in said projections so that the outlets thereof are either flush with the internal surfaces of the projections or slightly recessed in relation thereto. By mounting the nozzle assemblies in helical projections as described the nozzle assemblies are protected in case the wire breaks away during treatment and the wire cannot foul the ends of the nozzle assemblies as the wire is threaded through the chamber.

It is desirable to be able to use the apparatus for treating wire continuously and we have found that in some cases the thicker wires can be threaded through the chamber without the necessity of opening the latter.

Preferably, therefore, we provide a first funnel-shaped collector externally of the chamber and converging to the aperture means at one end thereof (i.e. the inlet end) and also in or on said chamber parts we provide a surface which provides a second funnel-shaped collector within the chamber and converging to the aperture means at the other end of the chamber.

When wire is being fed to the chamber, therefore, it is first directed into the first collector which guides the wire into one end of the chamber and as the end of the wire approaches the other end of the chamber it engages the second collector which leads the end through the second aperture means.

When using a tubular blasting chamber as described above for treating wire a further problem may arise. If the wire is being withdrawn from coil, which is the normal arrangement, and the wire on successive coils is not joined together by welding there will be a gap between the wire at the end of one coil and the beginning of the next coil. For satisfactory processing, both ends of each length of wire must be cleaned but, due to the manner of wire feed, one or other of the aperture guides at the ends of the chamber will have no wire passing through it for a short period at the changeover between coils. If the apertured guide is left open with no wire passing through it air will be able to enter the chamber and the intensity of blasting in the chamber will be reduced resulting in loss of blasting efficiency and inadequate cleaning of the end portion of the wire which passes through the chamber while one of the guides is open.

Preferably, therefore, we provide closure means for automatically closing each aperture means when the latter is not occupied by a workpiece.

Thus, for example, at the end of a coil, when the end of the wire has passed through the aperture means at the inlet end of the chamber the aperture means will be closed and the blasting of the end portion of the wire will proceed with full efiiciency until it is withdrawn from the chamber at the outlet end thereof. Similarly, the introduction of the leading end of another coil will open the aperture means at the inlet end of the chamber but the aperture means at the outlet end will be closed once the trailing end of the previous coil has left it. By this means, the blasting efficiency will be retained and the end portion of the wire will be properly cleaned.

Preferably, the closure means is arranged to be operated by the workpiece i.e. the wire itself. Thus the closure means may comprise pivoted flaps, the flap at the inlet end of the chamber being engaged by the leading end of the wire to open the aperture means and the flap at the outlet end of the chamber being held open by the wire passing through the aperture means at the outlet end until the trailing end of the wire is clear of the aperture means whereupon the flap will close.

The flap at the inlet end of the chamber may be mounted within the chamber itself and the flap at the outlet end of the chamber may be mounted in a tubular extension of the chamber arranged to reduce loss of vacuum from the chamber when the flap is opening or closing at the end of a coil.

The flap at the outlet end may be urged to its closed position by the vacuum in the chamber and the flap at the inlet end may be held closed by its own weight or by a balance weight.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described in detail by way of example with reference to the accompanying drawings in which:

FIG. 1 is an elevational view of the main part of an abrading apparatus embodying the invention with certain parts omitted;

FIG. 2 is a side elevation, partly in section of an abrading chamber forming part of the apparatus of FIG. 1;

FIG. 3 is an end elevation of the blasting chamber of FIG. 2 showing one part thereof in its open position in chain dotted lines;

FIG. 4 is a cross-section on the line 44 of FIG. 2 through the blasting chamber;

FIG. 5 is a plan view of the abrasive distributing means forming part of the apparatus of FIG. 1; and

FIG. 6 is a section showing the means for entraining the abrasive in the primary air.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring first to FIGS. 1 to 4, the apparatus shown comprises four parts, the blasting chamber indicated generally at 10 in FIG. 2, the means for separating reusuable abrasive indicated generally at 11 in FIG. 1, dust collecting means indicated generally at 12 in FIG. 1 and suction means indicated generally at 13 in that figure.

Referring first to FIGS. 2, 3 and 4 the blasting chamber 10 is tubular and comprises two generally channelshaped parts 14 and 15. The chamber is mounted on a base structure indicated generally at 16 and which comprises two end members 17 and 18 which are similar and which are made of square tube.

The end members have rigidly secured thereto plates 19 and 20 respectively and the part is rigidly secured between the plates 19 and as will hereinafter be described.

Each plate 19 and 20 carries a bearing housing 21 within which is mounted a pivot pin 22 rotatable in bearings 23. Each pivot pin 22 is secured to an arm 24 and the arms 24 carry the part 14. The arms 24 are secured to the pivot pins 22 and also secured to each of said pins is a collar 25 and a second arm 26. Extending between the free ends of the arms 24 and 26 is a pivot pin 27 which, as shown in FIG. 3, carries an eye member 28 which is connected to the piston rod 29 of a hydraulic jack 30. Upon operation of the two jacks 30, the arms 24 are moved from their full line positions in FIG. 3 to their chain-dotted line positions in that figure in which they are indicated at 24a and they carry with them the part 14 of the blasting chamber.

The blasting chamber has an inlet end housing 31 and an outlet end housing 32. The inlet end housing 31 is formed with an extension 33 having a funnel-shaped inner lining 34 of wear resistant material which is held in position by pins 35. The inner lining leads to a guide 36 of hard material to receive the wire to be passed through the blasting chamber. The passage 37 through the guide is normally closed by means of closure flap 38 which is pivoted at 39 in such a manner that the flap tends to remain closed under its own weight. The housing 31, the lining 34 and the guide 36 are each formed of two parts which, when the chamber 10 is closed as shown in FIG. 1, meet along the meeting plane between the parts 14 and 15. One part of each of the lining 34 and the guide 36 is secured to one part of the housing 31 and the other part of each of the lining 34 and the guide 36 is secured to the other part of the housing 31.

The outlet end housing 32 is provided with an inner lining 40 of hard material which is maintained in position by pins 41 and the inner lining is conical and leads to an apertnred guide 42 of hard material, for example, ceramic material, and the passage 43 through the guide 42 is normally closed by a closure flap 44 which is pivoted at 45 so that the flap tends to return to its closed position under the influence of its own weight. The housing 32 is provided with a tubular extension 46 which is provided with a further tubular guide 47 at its free end. The housing 32, the lining 40, and the guides 42 and 47 and the extension 46 are each formed of two parts which meet along the meeting plane between the parts 14 and 15 and one part of each of the lining 40, the guides 42 and 47, and the extension 46 is secured to one part of the housing 32 and the other part of each of the lining 40, the guides 42 and 47 and the extension 46 is secured to the other part of the housing 32.

Bolts 48 secure together, on the one hand the part 14 and arms 24 and one part of each of the housings 31 and 32 and, on the other hand, the part 15, the plates 19 and 20 and the other parts of each of the housings 31 and 32. Each bolt 48 carries a nut 48a, which holds the part 14 to the arm 24 or the part 15 to the plates 19 and 20 and a nut 48b which holds the end housing 31 or 32 in position. The end housings can thus be changed, eg to make the chamber suitable for treating wire instead of rod, and vice-versa, without disturbing the connection between the part 14 and the arms 24 on the one hand and the part 15 and the plates 19 and 20 on the other hand. Suitable, half-circular gaskets 49 are provided between the part 14 and the arms 24, between the part 15 and the plates 19 and 20 and between the arms 24 and the parts of the end housings 31 and 32.

It will be seen that by virtue of the split construction of the chamber 10 and the housings 31 and 32 with their associated parts, when the arms 24 are pivoted by the jacks 30 to their positions 24a the chamber is opened by moving the part 14 to the position 14a in FIG. 3 together with its associated parts of the end housings 31 and 32 with their associated inner parts. A wire or plurality of wires to be treated may then be threaded through and the chamber then closed.

The blasting chamber is provided with a plurality of nozzle assemblies and these are arranged in two helices about the longitudinal axis of the blasting chamber, the helices being spaced axially of the chamber by a small distance compared with the total length of the chamber.

Thus referring to FIG. 2 it will be seen that each nozzle assembly 50 lies opposite the space between two adjacent nozzle assemblies 51 and that each nozzle assembly 51 lies opposite the space between two adjacent nozzle assemblies 50. The nozzle assemblies are arranged so that their outlets are directed to the longitudinal axis of the chamber and they are arranged to pass through an external helical rib 52 on the parts 14 and 15.

Referring to FIG. 4, it will be seen that the external helical rib 52 corresponds to an internal helical rib 53. The nozzle assemblies are mounted so that their outlets are generally flush with the internal surface of the helical rib 53 for a purpose which will hereinafter be described.

Referring to FIG. 4, each nozzle assembly is similar and comprises a steel tube 54 having a lining 55 of synthetic rubber which has a convergent passage 56 therein. Fitting within a rebate 57 in the lining 55 is the upper end of a nozzle tube 58 which is formed of any suitable material, for example ceramic. The nozzle tube is received in a mounting 59 which in turn is received in the rib 53 and which is externally screw threaded at 68 and is split to receive a nut 61. The mounting receives the tube 54 and as the nut 60 is tightened the tube is clamped in position on the mounting 58.

The tube 54 is provided with a series of holes 62 which as will be described below permit the entry of tertiary air into the mixture flowing into the nozzle assembly, the lining 55 having recesses 63 into which the holes 62 open, the recesses being separated by ribs 64. A primary air inlet conduit in the form of a hose 65 is received within the upper end of the tube 54 and conveys a mixture of air and abrasive to the nozzle assembly.

Each nozzle assembly 50 and 51 is similar so that only one nozzle assembly has been described. It will be seen from FIG. 4, however, that the outlet end 66 of the nozzle tube 58 is substantially flush with, or slightly recessed relative to the inner surface 67 of the rib 53 so that the nozzle tubes do not project from the internal surface of the rib and, as will be described below, are protected from damage by any workpiece which may engage the rib 53.

The part of the chamber is provided with a large spigot 68 having a flange 69 thereon to which is connected a suction conduit 70.

As will be described hereinafter in more detail, air and abrasive is discharged from the nozzle tubes 58 against the workpiece passing through the blasting chamber between the guides 36 and 42 and the air and the abrasive together with any detritus produced as a result of the abrasive impinging on the workpiece are withdrawn through the suction conduit 70 and passed to the means for separating reusable abrasive indicated at 11.

The separating means 11 comprises a cyclone 75 to which the suction conduit 70 is connected at 76. The cyclone 75 separates reusable abrasive from the mixture of air and abrasive which it receives from the suction conduit 71 the reusable abrasive falling to the bottom of the cyclone as will hereinafter be described and the dust and air passing through a pipe 77 into a second cyclone 78 forming part of the dust collecting means. The dust falls to the bottom of the cyclone 78 and is received in a dust box 79 from which it may be removed. The air leaving the cyclone 78 passes through pipework indicated generally at 80 to the bottom of a bag filter 81 where any residual dust is separated out and received in a dust box 82, the substantially dust free air then passing through pipework 83 to an exhauster fan 84 driven by an electric motor 85. It will be seen, therefore, that the exhauster fan 84 applies suction to the suction conduit 71} via the cyclones 75 and 78 and the filter 81 and thus is responsible for drawing primary air and abrasive into the chamber 10.

The reusable abrasive which falls to the bottom of the cyclone 75 is deflected by a battle 86 and is received in an upper hopper 87. From the upper hopper 87 the abrasive can pass into a middle hopper 88 when the pressures in the two hoppers are equal. The pressure in the hopper 88 is varied by means of first valve means 89. Second valve means is interposed between the upper hopper 87 and the middle hopper 88, is indicated generally at 90 and comprises a flap 91 of hard rubber or similar material freely pivoted about its upper edge so as to be movable in response to the net resultant force exerted on it by the difference in the pressures between the hoppers 87 and 88 and any abrasive in the hopper 87 tending to open the flap 91. The middle hopper 88 is sealed at its top at 92 and is in air tight relation with the upper hopper 87.

An open topped, generally cylindrical lower hopper 93 is provided having within its base a conical baffle 94. Third valve means, indicated generally at 95 is interposed between the middle hopper 88 and the lower hopper 93, the third valve means comprising a flap 96 mounted in a manner similar to the flap 91 i.e. pivoted about its upper edge so that it will move in response to the net resultant force exerted on it by differences in the pressures between the hoppers 88 and 93 and the weight of any abrasive in the hopper 88 acting on the flap 96.

The first valve means 89 operates so that the middle hopper 88 is alternately placed at the pressure of the upper hopper 87 and at the pressure of the lower hopper 93 i.e. atmospheric pressure.

The operation of the first valve means may be as described in US. Patent No. 3,307,296 issued Mar. 7, 1967 or may be as described in Patent application Ser. No. 852,155 filed Aug. 19, 1969 of Denis Cyril Field and Madeleine Helen Field as Administrators of Anthony Gerard Field (deceased), the inventor and assigned to the Assignee of the present application. When the middle hopper 88 is at the pressure of the upper hopper 8 7, atmospheric pressure closes the third valve means 95 but the second valve means 90 can open to let abrasive pass from the upper hopper 87 to the middle hopper 88. When the middle hoper 88 is placed at atmospheric pressure i.e. the pressure of the lower hopper 93, the second valve means 90 is maintained closed by the atmospheric pressure acting on the flap 91 while the third valve means 95 can open to transfer abrasive from the middle hopper 88 to the lower hopper 93.

Abrasive delivered into the lower hopper 93 is guided by the bathe 94 to the lower peripherial portion of the hopper and from thence is entrained in input conduits formed by the hoses such as 65 described with reference to FIG. 4.

Referring to FIGS. 5 and 6 the base of the lower hopper is formed by a ring 97 to which is welded the lower edge of the baffle 94 and also the lower edge of the outer wall 98 of the hopper. In the ring 97 there is provided a plurality of circular threaded apertures 99 in each of which is threadedly received a nozzle member 100 through which the abrasive can pass at a metered rate.

Arranged beneath the ring 97 is a further ring 101 which has a plurality of cylindrical apertures 102 therein, the apertures 102 being coaxial with the apertures 99. Welded to the upper surface of the ring 101 coaxial with each aperture 192 is a sleeve 103 into which the nozzle member 100 directs abrasive. The base of each aperture 102 is closed by a plug 194-. The ring 101 also has a plurality of transverse apertures, one of which is indicated at 105, each transverse aperture intersecting an aperture 102 and threadedly receiving a union member 106 to which is clamped one end of a hose 65 which forms an inlet conduit, the other end of the hose 65 being connected to a nozzle assembly 50 or 51 as described in relation to FIG. 4. It will be appreciated that each of the nozzle assemblies 50 and 51 is supplied with air and abrasive along its own hose or primary air inlet conduit 65.

When suction is applied to the chamber 10, primary air is induced into the upper end of each sleeve 103 and draws abrasive out of the lower hopper 93 through the nozzle members 100. The mixture of primary air and abrasive 7 then passes along the hoses or primary air inlet conduits 65 to the nozzle assemblies 50 and 51 where the mixture is directed at the surface of a workpiece passing through the chamber and the abrasive and air is then returned along the suction conduit 70 as has been described above.

The operation of the apparatus as thus far described is as follows. The exhauster fan 84 is driven by the motor 85 so that the suction conduit 70 is placed under suction through the cyclones 75 and 78, the filter 81 and the pipework 77, 80 and 83. Suction applied to the suction conduit 70 removes air from the chamber 10. As the pressure in the chamber 10 decreases, primary air is forced to flow into the sleeves 103 and along the primary air inlet conduits 65 to the nozzle assemblies 50 and 51. The flow of primary air into the sleeves 103 induces abrasive to flow from the hopper 93 through the nozzles 100 into the sleeves 103 and the abrasive is mixed with the primary air and is transported by the primary air along the air inlet conduit 65 to the nozzle assemblies 50 and 51. The mixture of air and abrasive then enters the passage 56 in the lining 55 in each nozzle assembly and as the mixture flows past the holes 62 tertiary air is drawn in to mix with the primary air and abrasive and the mixture of pri mary and tertiary air and abrasive passes down the nozzle tubes 58 to be directed at a workpiece passing through the chamber in a manner to be described. The addition of tertiary air through the holes 62 accelerates the mixture of primary air and abrasive so that for a given speed of impingment of the air and abrasive on a workpiece the speed of the primary air and abrasive in the primary air inlet conduits 65 will be lower due to the provision of tertiary air that it could be were the provision of tertiary air omitted.

The blasting chamber 10 shown in FIG. 2 is intended primarily for treating wire or bar (hereinafter both referred to as wire) and the end of the wire is fed into the chamber at the left-hand end thereof in FIG. 2 and leaves at the right-hand end thereof in that figure. If the wire is rigid then it can be fed through the chamber without opening the latter. Due to its conical shape, the lining 34 in the extension 33 guides the end of the wire to the passage 37 in the guide 36 and the end of the wire displaces the closure flap 38 so that the wire can pass into the chamber 10. The wire is fed forward through the chamber and the end is collected by the lining 40 and fed to the guide 43 and displaces the closure flap 44 and then the end passes through the guide 47. If the wire is thin and is insufficiently rigid to be threaded through the chamber the latter is opened by operating the jacks 30, the wire led through and the chamber then closed. A number of lengths of thin wire can be passed through the chamber together.

The length of wire between the end housings 31 and 32 of the chamber can now be subjected to a blast of abrasive due to the direction of the nozzle assemblies 50 and 51 at the longitudinal axis of the chamber. As hasbeen mentioned above, we have found that by placing the nozzle assemblies 50 and 51 in two helices which are spaced apart a short distance from each other in the direction axially of the chamber a very uniform blasting pattern is formed and the wire is uniformly treated.

When the closure flaps 38 and 44 are open, the wire is guided between the guides 36 and 43 and these restrict the entry of secondary air into the chamber around the periphery of the wire. Assuming that the wire is being processed in a number of coils, when the leading end of a new coil is fed into the chamber the flap 44 will be closed and the flap 38 will be displaced by the leading end of the wire. By virtue of the flap 44 being closed this will help to maintain the vacuum in the chamber 10. The wire holds open the flap 38 and the entry of secondary air is controlled by the clearance between the wire and the passage 37 as described above. The leading end of the wire will then open the closure flap 44 as described above. When the trailing end of the wire enters the chamber 10,

the flap 38 will close automatically after the trailing end of the wire has passed and will thus retain the vacuum in the chamber as the last portion of the wire is being treated until as the trailing end leaves the chamber the flap 44 will close after it. The provision of the tubular extension 46 helps to reduce the loss of vacuum as the trailing end leaves the chamber and the closure flap 44 closes.

The vacuum in the chamber will tend to hold the closure flap 44 closed and the closure flap 38 is so constructed as described above that it will remain closed against the action of the atmospheric air on its outer surface under its own weight.

Should the wire move away from its central path, it cannot damage the nozzle tubes 58 due to the fact that these are recessed in the rib 53 so that the latter protects the nozzle tubes from the wire straying from its central path. We have found that with comparatively thick wire the funnel-like linings 34 and 40 guide the wire to the passages 37 and 43 in the guides 36 and 42, Moreover, the thicker wire is sufliciently rigid to open the flaps 38 and 44 as it is fed into and through the blasting chamber 10.

The abrasive, air and detritus leave the chamber 10 along the suction conduit 70 and pass through the cyclone where the reusable abrasive is separated out and passes through the hoppers 87, 88 and 93 by cycling of the first valve means 89 as will be described below until it is returned to the air inlet conduit 65 and it is thus recirculated to the nozzle assemblies 50 and 51. The dust and abrasive which is no longer usable is removed in the second cyclone 78 and in the bag filter 81 so that the air discharged to atmosphere by the exhauster fan 84 is substantially dust free.

The first valve means 89, acts so as cyclically to place the middle hopper 88 at the pressure of the lower hopper 93, in this case atmospheric pressure, and the pressure of the upper hopper so that the second and third valve means 90 and alternately open and close thus allowing abrasive to be transferred from the upper hopper to the lower hopper without unduly disturbing the low pressure in the system.

I claim:

1. Apparatus for abrading a surface on a workpiece comprising a tubular blasting chamber formed in two generally channel-shaped parts which are hinged together, a primary air inlet conduit connected at one end to the blasting chamber, means for entraining abrasive in the primary air flowing through the conduit upstream of its entry into the blasting chamber, aperture means at opposite ends of the chamber in the wall of the chamber to be closed by the workpiece, means for applying suction to said chamber to draw primary air and abrasive into the chamber and remove air and spent abrasive from the chamber when said aperture means is closed by the workpiece, means for directing abrasive entering the chamber against said surface and apertured guides for the workpiece for restricting the entry of secondary air into the chamber around the periphery of the aperture means when the latter is closed by the workpiece, and wherein the means for directing abrasive entering the chamber against the workpiece comprises a plurality of nozzle assemblies which are arranged so that their outlets into the chamber lie on at least two helices having coincident axes parallel to the longitudinal axis of the chamber, the helices being spaced apart axially of the chamber by a distance which is small compared with the total length of the chamber and each nozzle being directed at the axis of the helix on which the nozzle outlet lies, each helix comprising at least one full turn.

.2. Apparatus according to claim 1 wherein the nozzle assembly outlets on one helix lie opposite to the spaces bteween adjacent similar outlets on another helix.

3. Apparatus according to claim 1 wherein the nozzle assemblies are mounted on inward helical projection means on said parts of the chamber so that the outlets of said nozzle assemblies do not extend from said projection means.

4. Apparatus according to claim 3 wherein there is a single helical projection in which the outlets of the nozzle assemblies are located.

5. Apparatus according to claim 1 including a first funnel-shaped collector externally of the chamber and converging to the aperture means at one end thereof and a second funnel-shaped collector within the chamber and converging to the aperture means at the other end of the chamber.

6. Apparatus according to claim 1 including closure means for automatically closing each aperture means when the latter is not occupied by a workpiece.

7. Apparatus according to claim 6 wherein the closure means is arranged to be operated by a workpiece passing through the chamber.

8. Apparatus according to claim 7 wherein the closure means comprises pivoted flaps.

9. Apparatus according to claim 8 wherein the flap at the inlet end of the chamber is mounted within the cham- W ber itself and the flap at the outlet end of the chamber is mounted in a tubular extension of the chamber arranged to reduce loss of vacuum from the chamber when said flap is opening or closing.

10. Apparatus according to claim 8 wherein the flap at the outlet end of the chamber is urged to its closed position by the vacuum in the chamber and the flap at the inlet end of the chamber is held closed by its own Weight.

References Cited UNITED STATES PATENTS 2,415,844 2/ 1947 Perkins 51-8 3,286,406 11/1966 Ashworth 5 l-8 3,307,296 3/1967 Ashworth 51-8 LESTER M. SWINGLE, Primary Examiner 

